Here I have a pic of the wifi periodic drops http://i.imgur.com/JhRVP8F.png. Server is connected to the router per LAN. I'm connecting to the server through the router's WLAN. Bytes are transferred in one direction using nc. The graph at the top is the CPU load on the router, another one below is the router's wlan1(5GHz) iface throughput in MB/s. I thought that the router is guilty so I measured cpu and wlan iface statistics on the router. But it turned out that the problem lies on the client side and on the NetworkManager to be precise. As these drops happens exactly with 120s periodicity I assume that it's the background station scanning nm does even when connected to a station! To prove that the nm is to blame I deactivated it and activated the wlan iface manually. And here is how it looks like without nm - http://i.imgur.com/UWdUg4y.png

I consider this bug as major.

while fixing the issue please consider adding a rescan button to the nm-applet